oOii Variations of the Ttrrestrial Magnetism 



On seeing the inclinations of the compass so exactly re- 

 prestntcd in our hyuothesis, we endeavou'ed to discover 

 ■vvhether it could be applied also to the intensities observed 

 by M. Humboldt; but we found that it did not apply. It 

 gives, indeed, an increase of the magnetic forces from 

 the equator to the pole ; but thi? increase, which at first 

 is too slow, becomes after-wards loo rapid : I have not yet 

 been able to trv \\ hether the small displacemcrit of the ter- 

 restrial inagnet will contribute towards reprcsentino; them 

 better : but it must be remarked, that the series of The in- 

 tensities is extrenieiy whimsical, and contains an inlinite 

 number of anomalies ; so that local phasnomena may have 

 on this phsenomenon a much more sensible influence than 

 on the inclination. 



On reviewing the results which we have given in this 

 memoir, it is seen that we have' first determined the posi- 

 tion of the magnetic equator bv direct observations, which 

 had never been done before; we have then proved that the 

 magnetic force increases in proceeding from that equator to 

 the poles : in the last place, we have given a mathenratical 

 hypothesis, which when reduced to a formula satisfies all the 

 inclinations hithe'no observed. 



Supposing, as we have done in this formula, the small 

 corrections of which it is susceptible, its utility becomes 

 evident, either for making knou n, in the course of time, the 

 variations which may take place in the action of the terres- 

 trial masinetism, or to ascertain or even foresee the value of 

 the inclination, which in a great many cases is of great im- 

 portance. 



For example, near the magnetic equator, the increase or 

 diminution of the inclination will indicate to a vessel on a 

 voyage whether she has gained or lost in latitude by cur- 

 rents. Tliis knowledge of the latitude is sometinaes as im- 

 portant as that of longitude. On the coasts of Peru, for 

 example, the currents tend from Chiloe to the north and 

 north-east with such force, that one may go from Lima to 

 Guayaquil in three or four days, and two, thrte, and some- 

 tiuics live months are necessary to return. It is conse- 

 (juently of the greatest importance for vessels coming fronji 

 ' Chili which stretch along the coast of Peru, to know their 

 latitude. If th.ey go beyond' the port to which they are 

 bound they must work to the southward, and eyerv day's 

 progress requires often a month of return. Dnforiunatcly, 

 the fo::s which prevail during four or five iponths on the, 

 coasts of Peru prevent navigators from distinguishing the. 

 form of the coast; nothing is seen but the summits of the. 



AndeSj 



